Process for anodizing aluminum materials

ABSTRACT

A METHOD FOR PRODUCING AN ABRASION RESISTANT ANODIC COATING WITH A DESIRED CORROSION RESISTANT CHARACTERISTIC ON THE SURFACE OF ALUMINUM AND ALLOYS THEREOF COMPRISING THE STEPS OF: (1) INITIALLY ANODIZING THE ALUMINUM MATERIAL IN AN ACID BATH AT CERTAIN CONDITIONS OF TEMPERATURE, CURRENT DENSITY, VOLTAGE ETC., IN ORDER TO PRODUCE A DENSE, HARD ANODIC COATING; (2) SUBJECT THE MATERIAL TO ANOTHER ACID ANODIZING BATH AT CERTAIN TEMPERATURES, CURRENT DENSITY VOLTAGE, ETC., IN ORDER TO DEVELOP A CORROSION RESISTANT ANODIC COATING BETWEEN THE HARD ANODIC COATING AND THE BASE ALUMINUM MATERIAL THEREBY MINIMIZING THE EFFECTS OF THE MICROSPOPIC DISCONTINUITIES, AND THEN (3) SEALING.

May 30, 1972 F HARRlS EI'AL 3,666,638

PROCESS FOR ANODIZING ALUMINUM MATERIALS Filed April 21, 1970 INVENTORS,

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ATTORNEYL,

United States Patent O 3,666,638 PROCESS FOR ANODIZYING ALUMINUM MATERIALS Frank L. Harris, 3803 Massachusetts Ave. SE., Washington, D.C. 20019, and Sidney Levine, 8505 Buckhannon Drive, Potomac, Md. 20854 Filed Apr. 21, 1970, Ser. No. 30,405 Int. Cl. C23f 7/06 U.S. Cl. 204-35 N 6 Claims ABSTRACT OF THE DISCLOSURE A method for producing an abrasion resistant anodic coating with a desired corrosion resistant characteristic on the surfaces of aluminum and alloys thereof comprising the steps of:

(l) initially anodizing the aluminum material in an acid -bath at certain conditions of temperature, current density, voltage etc., in order to produce a dense, hard anodic coating;

(2) subject the material to another acid anodizing bath at certain temperatures, current density, voltage, etc., in order to develop a corrosion resistant anodic coating between the hard anodic coating andthe base aluminum material thereby minimizing the effects of the microscopic discontinuities, and then (3) sealing.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to a process for producing a hard, corrosion resistant surface on aluminum and alloys thereof.

In the field of illumination, aluminum has long been recognized for its ability to reflect White light. However, the use of aluminum as a reflector was. curtailed by the loss of reflectivity over an extended period of time due to industrial atmospheric influences and yweathering and to the susceptibility of the reflector surface to scratching, when cleaned.

In the early l930s the sulfuric acid anodic oxidation process made its commercial debut in the United States, thereby permitting the formation of a protective coating of aluminum oxide on the aluminum surface. This development overcame the loss of light output due to corrosive influences and resistance to surface abrasion.

The application of the anodic coating accomplished the desired results. However, other problems were introduced. For example, the initial reflection factor of the aluminum was lowered. However, new chemical and electrochemical brightening procedures were developed. With the introduction of theseA processes, considerable interest in brightf'inished and anodically treated aluminum developed in areas outside the eld of illumination, eg., automotive and appliance trim, refrigerator shelves, architectural applications, and continuously processed coiled stock for subsequent fabrication. The aluminum industry responded by the introduction of alloys receptive to the brightening and anodic processing technique for stamping, extrusion, spinnings, machined parts, forgin gs, etc.

The use of hard-anodized aluminum is precluded in many applications where extremely corrosive environments exist because of the crack system which extends from the coating surface to the base metal. This crack system originates because of differences between the coefficients of thermal expansion of the metal and the hardanodic coating. Another disadvantage of hard-anodized aluminum is the lack of sufficient coverage at sharp edges such as corners and threaded areas.

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It was considered that the corrosion resistance of hardanodized aluminum or alloys thereof could be significantly increased by plugging the microscopic discontinuities so that there would be no free pathways to the base metal. It was deemed practical to close the crack system by de- -velopment of an anodic coating between the metal and' the hard coat after development of the hard coat. This composite coating provides an effective barrier at points where the cracks are open to the base metal.

The materials which may be beneficially treated utilizing the present method comprises aluminum and aluminum based alloys including alloying elements copper, zinc, magnesium and zinc. These alloys include the structural and nonstructural alloys. An example of nonstructural alloys is aluminum 6061 composed of aluminum and magnesium. Examples of structural alloys, i.e., having the ability to withstand stress and strain, are aluminum 7075 and 2024 composed of aluminum and zinc, and aluminum and copper, respectively. Examples of items composed of structural alloys are automobile pistons. Thus, it is apparent that many objects composed of aluminum and its alloys must have the ability to withstand both the effects of corrosion and abrasion.

The treatment of aluminum utilizing a hard anodizing process produces a dense, hard-anodic coating on the treated materials having microscopic discontinuities extend to the metal and therefore are the means whereby corrosive materials come in contact with the metal and thereby cause the corrosion thereof. To counteract the corrosive effect, the hard-anodized material is subjected to a sulfuric acid anodizing process which results in the development of a corrosive resistant oxide lm beneath the first coat, i.e., between the hard coat and the base aluminum. The second anodic coating is able to permeate the first or outer coating of aluminum oxide due to the porous nature thereof. The material containing the resultant composite coating is subsequently immersed in boiling water to effect sealing, thereby resulting in a coating which is completely continuous. This results in a surface which is both corrosion and abrasion resistant.

It is an object of this invention to provide and disclose an improved process for anodizing aluminum.

It is a further object of this invention to provide and disclose an improved process for anodizing aluminum alloys.

It is a further object of this invention to provide and disclose an improved process for anodizing aluminum materials comprising the subjection of aluminum materials to two distinct acid baths in selected order to produce a composite coating which is completely continuous and then sealing.

Other objects and a fuller understanding of this invention may be ascertained by referring to the following description and claims taken in conjunction with the accompanying drawing in which:

FIG. 1 shows an illustration of the system utilized in the anodizing process.

FIG. 2 shows a sectional view through 2-2 of FIG. l.

FIG. 3 shows a ow chart of the steps of the anodizing process.

Referring now to FIG. l, the system comprises tank 10. The exterior of the tank may be constructed of any suitable material, e.g., plastic. The interior of tank 10 is lined with lead 13, as illustrated in FIG. 2, which also serves as the cathode. The tank may be of any suitable dimensions depending upon the size of the aluminum materials to be treated. A suitable mineral acid electrolyte, eg., sulfuric acid 15, is contained within lead lined tank 10. Aluminum material 17 is immersed into the electrolyte. Said material may be supported by any suitable means, eg., traverse-beam 19. Said beam may be constructed of aluminum or titanium. Aluminum is more economical and has higher electrical conductivity, but is anodized along with the parts being processed, and therefore requires stripping before reracking. Titanium is not anodized, but is much more costly initially and has much lower current-carrying capacity. Conduit means 21 are provided near the bottom of tank for the 'admittance of compressed air for agitation and cooling. Source 23 supplies the necessary electrical power to decompose the electrolyte. In addition tank 10 may lbe provided with iron pipe coils (not shown) for heating and cooling purposes.

The aluminum panel acts as the anode in the anodizing process. Utilizing a H2504 bath, the following reaction is considered to take place at the cathode thereby liberating H2: 4H3O++4E 1 4H2O+2H2. The following reaction is considered to take place at the anode thereby liberating O2: 40H -"2H20i02|-4E. The 02 combines with metallic aluminum to form aluminum oxide.

In operation, the aluminum materials are lirst subjected to a cleaning process as indicated in FIG. 3. The proper cleaning cycle and cleaning material will depend on several factors, e.g., the type of final finish desired, the amount of soil and the kind of soil. If it is necessary to remove unusual accumulation of soil, auxiliary cleaning, i.e., vapor degreasing or spray washing should be performed prior to the first anodizing operation.

In the present invention, a conventional hard anodize coating is first applied to an aluminum alloy, e.g.,` 6061, material. An electrolyte consisting of a by Weight H2504 aqueous solution was utilized. Prior to use, the solution was saturated with carbon dioxide. The operating conditions utilized were:

Temperature: 25-32 F. Current density: 25 amp/ft.2 Voltage: Up to 75 volts.

The process was carried out for a period of 160 minutes. Vigorous agitation utilizing compressed air was applied in order to prevent local overheating. An oxide coating of approximately 0.001 to 0.002" was obtained. An acid range of 12-25 by weight and a temperature range of 20-50 F. has been found operable. In addition to sulfuric acid, oxalic acid, alone or in combination with the sulfuric acid, may be utilized.

The hard anodized aluminum 6061 is rinsed and then subjected to a second anodizing bath. An electrolyte consisting of 15% by weight of a H2504 aqueous solution was used. The operating conditions utilized were:

Temperature: 70 F. 1 Current density: 15 amp/ft.z Voltage: 19 volts.

It should be understood that the specific values cited above are not restrictive, but are determined by coating requirements in each instance. Operable conditions include, e.g., temperature 60-80 F., current density 10-15 amp/ft.2 and voltage 14-19 volts.

The anodizing process was continued for a period of 40-50 minutes. A reaction period of e.g., 15-50 minutes may be utilized. A resultant maximum coating of up to 0.0008" is desired. An acid range of 15-25% by Weight has been found operable. In addition to sulfuric acid, oxalic, alone or in combination with sulfuric, chromic, boric, phosphoric and sul-famic may be utilized.

It is pointed out that both of the acid bath steps and the sealing step of this new method are separately disclosed'in the prior art, i.e., Metal Finishing 1969 Guidebook Directory. Therefore no novelty is claimed in the separate steps of the method. The patentable novelty is considered to reside in the combination of the steps in a selected consecutive order in the treatment of an aluminum material to obtain a hard anodized finish with greatly improved corrosion resistance and unirnpaired abrasion resistant. Furthermore, the invention is not limited to the above disclosed specific acid baths.

The above treated aluminum 6061 is then subjected to a post-anodic treatment in order to provide enhanced corrosion resistance. The term sealing is used in the trade to describe various post-anodic treatments. The usual method of sealing comprises the immersion of the anodic coating in a hot or boiling-water bath in order to provide hydration of the anodically formed oxide.

The following reactions are considered to take place during sealing, (1) the formation of the aluminum oxide monohydrate, which is illustrated -by the following equation:

A1203 H209 A1203 and (2) the formation of aluminum oxide trihydrate, which is illustrated by the following equation:

Generally, it has been discovered that if the sealing process is carried out at an approximate neutral pl-l and at a temperature above C., the formation of the tmonohydrate is favored. However, at temperatures below 80 C., the formation of the trihydrate is favored. The monohydrate is preferred from a protective standpoint due to its lower solubility in various corrosive environments.

The 6061 aluminum alloy material containing the composite coating was sealed by the immersion thereof in boiling water having an approximate neutral pH for a period of 15 minutes, thereby forming the monohydrate.

Experimentation was conducted in order to show the benecial eiects resulting from the utilization of the aluminum materials which have been treated in accordance with the present process. Three metal objects, each consisting of the combination of a stainless steel threaded nipple and an aluminum fitting (6061) therefor, were selected. The stainless steel and aluminum combination was selected because of its propensity to corrode under adverse condition. Combination #l was treated in accordance with the heretofore described abrasion resistant sulfuric acid anodizing process and sealed. Combination #2 was treated in accordance with the heretofore described corrosion resistant sulfuric acid anodizing process and sealed. Combination #3 was treated in accordance with both the abrasion resistant sulfuric acid anodizing bath and the corrosion resistant sulfuric acid anodizing baths and sealed. The treated combinations were then placed in a 5% salt fog chamber for a period of 150 hours. The metal materials were then removed, allowed to dry, and examined. Substantial corrosion was evident on combination #l and #2. sIn addition, the aluminum fittings were frozen to the stainless steel threaded nipple. Combination #3 showed slight evidence of corrosion. In addition, the tting could be readily removed from the threaded nipple.

Although we have described our invention with a certain degree of particularity, we wish it to be understood that We do not desire to be limited to the exact details shown and described, for obvious modification will occur to a person skilled in the art.

Having described our invention, -we claim:

1. A process for anodizing aluminum and alloys thereof comprising the steps of: (l) treatment of an aluminum material in a rst aqueous hard anodizing acid bath under operating conditions of:

Temperature: 20-50 F.

Current density: 25 amp/ft.2

,Voltagez up to 75 volts for a period of 20-160 minutes in order to produce an abrasion resistant anodic surface, rinsing, treatment of said aluminum material in a second aqueous'corrosion resistant anodizing acid bath under operating conditions of:

Temperature: 60-80 F.

Current density: 10-25 amp/ft.2

Voltage: 14-18 volts for a period of 15-50 minutes such that a continuous anodic coating develops beneath the abrasion resistant anodic coating, and sealing.

2. A process in accordance With claim 1 wherein the hard anodizing acid bath is selected from the group consisting of sulfuric acid, oxalic acid, and a mixture thereof.

3. A process in accordance with claim 1 wherein the corrosion anodizing acid bath is selected from the group consisting of sulfuric, oxalic, chromic, boric, phosphoric, sulfamic, and a mixture of sulfuric and oxalic acids.

4. A process in accordance with claim 1 wherein the treated material is aluminum.

5. A process in accordance with claim 1 wherein the treated material is an aluminum based alloy containing alloy elements selected from the group consisting of copper, zinc, magnesium, manganese and silicon.

6. A process in accordance with claim 1 wherein the resultant composite coating is sealed by the immersion thereof in boiling water for a period of 15 minutes.

References Cited UNITED STATES PATENTS I OHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. C1. X.R. 204-42 

